Eggs in faeces and urine, hatch releasing miracidia, miracidia penetrate snail tissue, sporocyts in snail (successive generation), cercariae released by the snail into water. These are free swimming and penetrate the skin.

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What happens once the cercariae penetrate the skin?

They lose tails and become schistosomulae. Circulation occurs. These migrate to portal blood in liver and mature into adults. The paired adult worms migrate to mesenteric venules of bowel/rectum, laying eggs that circulate to the liver and shed in st

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What is the life cycle within the human host?

The cercariae lose tails during penetration and become schistosomulae. Circulation occurs.These migrate to portal blod in lover and mature. The paited adult worms migrate to mesenteric venules of bowel/rectu,. Leying eggs that circulate liver, shed.

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What is the last stage of the human host cycle?

Venous plexus of bladder

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What are adult worms like?

Dioecious and sexually dimorphic, live within the venous system, form pair (the female living in the male's canal), live for 5 yeats.

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Where are S.haematobium found?

They live in the vesicle plexus surrounding the bladder - urinary schistosomiasis.

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Where are other schistosome species found in the human body?

They live in veins of the mesenteric plexus surrounding the small and large intestines - intestinal schistosiasis

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How many eggs do schistosomes release throughout adult hood?

300-3000 per days

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What is the period of time between infection of human host to beginning of eggs laying by adult worms

25 - 30 days

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The eggs of most species of schistosome have what?

A characteristic spine - this is important for species identification and diagnosis.

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What is the process of the schistosomes entering the snail?

Eggs in faeces and urine in the water. The eggs hatch releasing miracidia. The miracidia penetrate into snail tissue, and become sporocysts (successive generations) Cercariae released into water, penetrate skin of human.

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What conditions do Miracidia eggs hatch in?

Freshwater

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What are the features of Miracidia?

Free living and motile, infective for snails 6-8 hours after hatching. Able to locate suitable hosts using external stimuli such as light and snail derived chemicals.

Released by the intermediate snail host, they are freel iving (survive up to 48 hours. They are very motile - using a forked tail to swim. Use water turbulence and skin derived chemical to locate humans. Penetrate the skin

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What do they become once they lose their tails?

Schistosomula

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What are the advantages of secondary hosts?

Increased reproductive potential - asexual reproduction can take place in alternative host. Increases range of parasie in space and time - infecting more than one host species parasite can survive when one host is scarce.

Infections rise rapidly through childhood, it peaks at older children/young adults/ declines with old age. There is an age releated pattern of infectio (based on field, experimental and theoretical studies)

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What is the pattern?

Children become infected when they are old enough to play in water, due high exposure to water. They are non immune so rapidly become infected.

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Why does infection decrease with age?

Adults acquire more immunity but also are less likely to be in contact with the water.

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Immunity transplantation experiments:

If you give a money immunised against mouse proteins mouse proteins, it will be immune. But it isn't immune against monkey proteins.

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What is concominant immunity?

It describes the situation where a primary worm burden persists while the host is resistant to a secondary challenge.